It is since long known that in, above all, the mounting of electrical motors comprising permanent magnets, the risk of the operator and/or the electrical motor being injured/damaged is imminent as a consequence of the large magnetic forces acting between the stator and rotor of the motor during mounting.
A known way to mount a submersible machine, for instance a pump for waste water, comprising an electrical motor that does not comprise permanent magnets includes the following steps. First, the pump housing, or the stator housing, is placed upside-down on a plane and stable surface. Next, the stator is mounted in the stator housing, which together form a stator unit. Next, the mounting of the rotor unit (drive shaft, rotor, bearing, and bearing housing) to the stator unit is carried out by having the rotor unit lowered into the stator unit, by means of a crane, or manually, and the guiding of the rotor unit in the transverse direction is made by hand, until the drive shaft and upper drive shaft bearing are in contact with the stator housing, i.e., until the fully mounted position is reached. Next, the oil housing and the hydraulic unit are mounted. It should be pointed out that many service workshops are not equipped with more sophisticated equipment than a crane and different hand tools.
However, it has become more and more common that submersible machines comprise electrical motors comprising permanent magnets, and this entails that the mounting and the dismounting will be cumbersome, or even impossible, as a consequence of the omnipresent and invisible magnetic attraction between the rotor core and the stator unit.
If the above-mentioned mounting steps are carried out on a pump having an electrical motor comprising permanent magnets, the mixer as well as the operator risk being damaged/injured; moreover it is doubtful if the mounting at all will be successful. The mounting begins with the stator unit placed on a plane surface. When the rotor unit comprising permanent magnets after that is lowered, by means of a crane or the like, and at a point immediately above the stator, the magnetic attraction is so strong that the stator unit will be lifted from the surface, whereupon the stator unit may fall down and risk being damaged and simultaneously risk injuring the operator.
Theoretically, the stator unit can be clamped to the support to prevent the same from being lifted when lowering the rotor unit. However, the rotor unit can still not be centred above the stator, but will uncontrollably swing until the rotor contacts and abuttingly adheres to the stator unit. The only applicable method in a service workshop to solve the centring of the rotor unit is then to knock on the rotor unit using a hammer until the same is centred. However, this method is not recommendable since the electrical motor, and above all the permanent magnets, risk being damaged.
Even if the operator manages to centre and insert the rotor unit into the stator unit, the next risk arises in that the rotor unit in a literal sense will be sucked into the stator unit by great force and a cutting action will arise between the stator housing and the bearing housing of the rotor unit, whereby the operator risks hurting or losing his/her fingers.
U.S. Pat. No. 6,919,659 B2 discloses a method for the mounting of a stator on a rotor unit comprising at least one permanent magnet. The disclosed method overcomes the problem of the stator contacting the rotor. The rotor is arranged in a plastic casing that is pressed into the stator during the mounting.
US 2007/0235248 A1 describes a method for the mounting of a stator on a rotor unit comprising permanent magnets. The described method overcomes the problem of the stator contacting the rotor. The method uses an outer casing that is fixedly connected to the rotor unit, and then the stator unit is lowered into the gap between the outer casing and the rotor. Thus, the method uses the outer casing as a guidance for the stator unit.
Neither of these documents provides a method that solves all problems associated with the mounting of an electrical motor comprising permanent magnets.